1. Field of the Invention
The invention relates in general to the fabrication of a metal-oxide-semiconductor device (MOS), and more particularly to the method of forming a MOS device, having an air-gap between the gate electrode and the spacer.
2. Description of the Related Art
In order to avoid short-channel effect, lightly doped drain (LDD) structure is frequently utilized in a MOS device. Referring to FIG. 1, LDD structure 11 contains a lightly doped region located at a region between the source/drain region 15 and the gate electrode 12 at the substrate 10 surface. However, the existence of the spacer 13 between the gate electrode 12 and the LDD region 11 results in an overlap capacitor 14, which is easy to cause the misjudgement during data access.
To reduce the overlap capacitor forming between the gate electrode region and LDD region, material with lower dielectric constant is frequently utilized as spacer. Besides, a MOS having an air-gap region between the gate electrode region and the spacer is proposed to prevent the formation of the overlap capacitor. Referring to FIG. 2, a MOS device 20 includes a gate electrode region 21, source/drain regions 22, a LDD region 23 and a spacer 24. The spacer 24 and the gate electrode region 21 are not directly connected but isolated by an air gap region 25. Therefore, the overlap capacitor between the gate electrode region 21 and the LDD region 23 is reduced.
However, the MOS device with the air-gap structure can not completely avoids the formation of the overlap capacitor since the spacer partially contacts with the LDD region.